1. Field of the Invention
The present invention relates to a semiconductor device and a rectifier system having such a semiconductor device.
2. Description of Related Art
In three-phase generators and dynamos, AC bridges or rectifiers are used for rectification. In general, the rectifiers are composed of six semiconductor diodes having a silicon pn junction. These diodes are designed for operating at high currents (e.g., current density up to 500 A/cm2) and high temperatures (e.g., junction temperature Tj<225° C.). The voltage drop in the direction of flux, forward voltage UF, is typically approximately 1 volt for the high currents used. When operating in the reverse direction, in general only a very small blocking-state current IR flows up to a breakdown voltage UZ. Starting at this voltage, the blocking-state current increases strongly. This prevents a further increase in the voltage.
A distinction is made between high-blocking diodes (HS diodes) having a UZ approximately in the range between 200 V and 400 V and Z diodes having blocking-state voltages of approximately 20 V to 40 V, depending on the electric system voltage of the motor vehicle. The high-blocking diodes (HS diodes) must not be operated in breakdown mode. Z diodes may also be operated in breakdown mode, briefly even at very high currents. They are therefore used for limiting the overshooting generator voltage in the event of load variations or load dump.
The forward voltage of the pn diodes which results in conducting-state power losses and therefore in impaired generator efficiency represents a certain disadvantage. Since on the average always two diodes are connected in series, the average conducting-state power losses in the case of a 100 A generator are approximately 200 W. The associated heating of diodes and rectifiers must be mitigated by using complex cooling measures (heat sinks, fans).
To reduce the conducting-state power losses, the present invention provides for the use of Schottky diodes (Schottky Barrier Diodes, SBD) instead of pn diodes. Schottky diodes are metal-semiconductor junctions which have an electrical characteristic similar to that of pn diodes. Contrary to pn diodes, in the case of Schottky diodes the forward voltage may be freely selected within certain limits by selecting the metal and, more importantly, may be adjusted to values that are lower than in the case of pn diodes. Thus, for example, forward voltages UF of 0.5 V to 0.6 V may be implemented without difficulty. The selection of the metal essentially defines the (power) barrier height PhiBn. Only electrons capable of overcoming this barrier contribute to the current flow. The barrier height may be influenced by a suitable selection of the “barrier metal.” In addition, barrier height PhiBn is also a function of the semiconductor used (semiconductor material: covalent or ionic semiconductor, n or p doping, etc.).
The relationship between forward current density jF and blocking-state current density jR is described by the following formula (see for example, the textbook “Power Semiconductor Devices” by B. J. Baliga, PWS Publishing Company, Boston, 1996):
                              j          ⁢                                          ⁢          R                =                  j          ⁢                                          ⁢                      F            ·                          exp              ⁡                              (                                                                            -                      q                                        ·                    UF                                                        k                    ·                    Tj                                                  )                                                                        (        1        )            where q is the elementary electric charge and k is Boltzmann's constant. Tj is the junction temperature of the diode in Kelvins. It is apparent that the blocking-state currents are high when the forward voltage is selected to be small. However, the blocking-state current may be selected to be sufficiently small for use in the generator by a suitable choice of UF. (Trade-off between forward voltage and blocking-state current.)
The above formula (1), however, describes only the boundary case of an ideal Schottky diode. In addition to the technology-based shortcomings, this formula does not describe, in particular, the relationship between the blocking-state current and the blocking-state voltage. This relationship causes the blocking-state current to increase far beyond the current values described by formula 1 when the blocking-state voltage increases. This is because of a reduction in the barrier height PhiBn with increasing blocking-state voltage. This relationship is also known as image force effect or Schottky effect or barrier lowering effect (BL). The blocking-state power losses generated by the blocking-state currents in conventional Schottky diodes are, in particular at the high temperatures occurring in the use of generators, in general so high that the system becomes thermally unstable.
Schottky diodes may in principle also be operated in blocking-state voltage breakdown, i.e., as Z diodes. Since the Schottky contact is located on the surface of the semiconductor, it is somewhat prone to crystal imperfections and contamination. Therefore, reliability in Z operation is often limited and, in general, is not allowed.
The semiconductor device of the generic type is a combination of a conventional Schottky diode with another semiconductor structure designed using trench technology and has a trench structure. The semiconductor device is also well suited as a Zener diode and may be used for use in a rectifier for a generator in a motor vehicle.
Published German patent document DE 694 28 996 describes semiconductor devices having structures including a combination of conventional Schottky diodes (SPD) with another semiconductor structure, namely a magnetoresistor, having a trench structure. The structure is composed of a plurality of cells. This type of Schottky diode is also known as Trench MOS Barrier Schottky (TMBS) diode.